Hybrid roller-mill bit and hybrid roller-drag bit

ABSTRACT

A roller-mill bit for use in a wellbore includes: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller disks, each roller disk secured to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller disk; and a fixed mill mounted to the bearing shafts and comprising a pad for each roller disk. Each pad is dressed with a cermet material.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure generally relates to a hybrid roller-mill bit and hybrid roller-drag bit.

Description of the Related Art

U.S. Pat. No. 2,320,136 discloses a composite rotary drilling bit having side roller gauge or reaming cutters for removing the outer portions of formation material from the hole bottom and also intermediate drag cutters for removing the central portion of material from the bottom inside of the annular path traced by the side cutters.

U.S. Pat. No. 5,853,055 discloses a rotary cone bit for drilling bore holes in earth formations whose body has a thread pin end and a dome end from which extend three legs. A cutter cone is rotatably mounted to each leg and is radially oriented about the bit's central axis. Each cutter cone has a gage row of cutting elements extending from the cone surface nearest the mouth and a nose row extending nearest the cone's apex. A center jet for emitting fluid or mud is located on the dome. The jet has a converging nozzle with an exit orifice which extends below a predefined horizontal plane intersected by the cones or cutting elements. The exit orifice has a constant diameter for a length at least equal to its diameter for reducing the diffusion of the fluid or mud flow emitted. Fluid or mud emitted from the center jet travels substantially uninterrupted within a cylindrical space between the cones which is not invaded by any cutting element. This reduced diffusion substantially uninterrupted fluid flow strikes the bore hole bottom with maximum impact energy for enhanced removal of earth formation cuttings.

U.S. Pat. No. 6,581,702 discloses a three-cone rock bit employing a non-plugging center jet nozzle with a plurality of staggered inlet orifices leading to side passageways to reduce bit balling. The nozzle defines a tapered cavity through which drilling mud flows and exits in streams. Streams are directed from the nozzle through a main exit aperture of sufficient size to avoid plugging and from side passageways boring through a sidewall of the nozzle. Jetting streams promote washing of voids within the bit and of cutting surfaces. The nozzle uses staggered inlet orifices leading to side passageways, in conjunction with a tapering shape of a central passageway to facilitate maintenance of drilling mud velocity within the central passageway and thus of stream velocity to targeted regions of the drill bit.

U.S. Pat. No. 7,845,435 discloses a hybrid drill bit having both roller cones and fixed blades and a method of drilling. The cutting elements on the fixed blades form a continuous cutting profile from the perimeter of the bit body to the axial center. The roller cone cutting elements overlap with the fixed cutting elements in the nose and shoulder sections of the cutting profile between the axial center and the perimeter. The roller cone cutting elements crush and pre- or partially fracture formation in the confined and highly stressed nose and shoulder sections.

U.S. Pat. No. 8,678,111 discloses a hybrid earth-boring bit including a bit body having a central axis, at least one, preferably three fixed blades, depending downwardly from the bit body, each fixed blade having a leading edge, and at least one rolling cutter, preferably three rolling cutters, mounted for rotation on the bit body. A rolling cutter is located between two fixed blades.

U.S. Pat. No. 9,353,575 discloses an earth boring drill bit, the bit having a bit body having a central longitudinal axis that defines an axial center of the bit body and configured at its upper extent for connection into a drillstring; at least one primary fixed blade extending downwardly from the bit body and inwardly toward, but not proximate to, the central axis of the drill bit; at least one secondary fixed blade extending radially outward from proximate the central axis of the drill bit; a plurality of fixed cutting elements secured to the primary and secondary fixed blades; at least one bit leg secured to the bit body; and a rolling cutter mounted for rotation on the bit leg; wherein the fixed cutting elements on at least one fixed blade extend from the center of the bit outward toward the gage of the bit but do not include a gage cutting region, and wherein at least one roller cone cutter portion extends from substantially the drill bit's gage region inwardly toward the center of the bit, the apex of the roller cone cutter being proximate to the terminal end of the at least one secondary fixed blade, but does not extend to the center of the bit.

US 2016/0348440 discloses a drill bit including a bit body having a longitudinal bit axis extending there through, a plurality of journals extending from the bit body, each journal having a journal axis extending from a base of the journal through the length of the journal, a roller cone rotatably mounted to each of the journals, and at least one blade protruding from the bit body center and extending radially outward to less than an outer diameter of the drill bit.

SUMMARY OF THE DISCLOSURE

The present disclosure generally relates to a hybrid roller-mill bit and hybrid roller-drag bit. In one embodiment, a roller-mill bit for use in a wellbore includes: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller disks, each roller disk secured to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller disk; and a fixed mill mounted to the bearing shafts and comprising a pad for each roller disk. Each pad is dressed with a cermet material.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

FIG. 1 illustrates a hybrid roller-mill bit positioned for drilling out a frac plug set in a wellbore, according to one embodiment of the present disclosure. FIGS. 2A-3B illustrate the roller-mill bit. FIGS. 4A-4C illustrates a cutter of a fixed mill of the roller-mill bit. FIGS. 4D-4F illustrate mounting of the cutter to a mill pad of the hybrid bit.

FIG. 5 illustrates drilling of a wellbore using a hybrid roller-drag bit, according to another embodiment of the present disclosure. FIGS. 6 and 7A illustrate the roller-drag bit. FIG. 7B illustrates mounting of a fixed cutting structure to the roller-drag bit.

FIGS. 8 and 9A illustrate a second hybrid roller-drag bit, according to another embodiment of the present disclosure.

FIG. 9B illustrates a third hybrid roller-drag bit, according to another embodiment of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates a hybrid roller-mill bit 1 positioned for drilling out a frac plug 2 set in a wellbore 3, according to one embodiment of the present disclosure. For a hydraulic fracturing operation, the frac plug 2 is set against a casing or liner string 4 to isolate a zone (not shown) of a formation adjacent to the wellbore 3. To set the frac plug 2, a setting tool (not shown) and the frac plug 2 may be deployed down the casing or liner string 4 using a wireline (not shown). The frac plug 2 may be set by supplying electricity to the setting tool via the wireline to activate the setting tool. A piston of the setting tool may move an outer portion of the frac plug 2 along a mandrel 5 of the frac plug while the wireline restrains a mandrel of the setting tool and the plug mandrel, thereby compressing a packing element 8 and driving slips 6 along respective slip cones 7 of the frac plug. The packing element 8 may be radially expanded into engagement with the casing or liner string 4 and the slips 6 may be wedged into engagement therewith.

The casing or liner string 4 may then be perforated above the set frac plug 2 and the isolated zone may be hydraulically fractured by pumping a ball 9 followed by fracturing fluid (not shown) down the casing or liner string 4. The ball 9 may land in a seat of the plug mandrel 5, thereby forcing the fracturing fluid into the zone via the perforations. Another frac plug (not shown) may then be set above the fractured zone and the casing or liner string 4 may again be perforated above the plug for hydraulic fracturing of another zone. This process may be repeated many times, such as greater than or equal to ten or twenty times, until all of the zones adjacent to the wellbore 3 have been fractured.

After all of the zones have been fractured, a production valve at the wellhead may be opened to produce fluid from the wellbore in an attempt to retrieve the balls 9. However, this attempt often fails. The roller-mill bit 1 (only partially shown) may be deployed down the casing or liner string 4 using coiled tubing (not shown). A drilling motor (not shown), such as a mud motor, may connect the roller-mill bit 1 to the coiled tubing. The roller-mill bit 1, drilling motor, and coiled tubing may be collectively referred to as a mill string. Milling fluid may be pumped down the coiled tubing, thereby driving the drilling motor to rotate the roller-mill bit 1 and the roller-mill bit may be advanced into engagement with the frac plug 2, thereby drilling out the frac plug. Once drilled out, the mill string may be advanced to drill out the next frac plug 2 until all of the frac plugs have been drilled out.

Alternatively, the mill string may include a string of drill pipe instead of coiled tubing with or without the drilling motor. Alternatively, the roller-mill bit 1 may be employed to drill out other types of downhole tools, such as packers, bridge plugs, float collars, float shoes, stage collars, guide shoes, reamer shoes, and/or casing bits.

FIGS. 2A-3B illustrate the roller-mill bit 1. The roller-mill bit 1 may include a body 10, a plurality of roller disks 11 a-c, a plurality of crushers 20, and a fixed mill 12. The roller disks 11 a-c, crushers 20, and fixed mill 12 may form a lower cutting face of the roller-mill bit 1. The roller disks 11 a-c and crushers 20 may be located at an outer portion of the cutting face and the fixed mill 12 may be located at an inner portion of the cutting face.

The body 10 may have an upper coupling 13, a lower leg 14 a-c for each roller disk 11 a-c, and a dome 15 formed between the legs. The body 10 and the roller disks 11 a-c may each be made from a metal or alloy, such as steel. The body 10 may be made by attaching three forgings together, such as by welding. The legs 14 a-c may be equally spaced around the body, such as three at one hundred twenty degrees. The upper coupling 13 may be a threaded pin for connection to the drilling motor or drill pipe. A bore may be formed through the coupling 13 and extend to a plenum (not shown) formed adjacent to the dome 15.

Each leg 14 a-c may have an upper shoulder 16 s, a mid shirttail 16 h, a lower bearing shaft 16 b, and a ported boss 16 p. The shoulder 16 s, shirttail 16 h, ported boss 16 p, and bearing shaft 16 b of each leg 14 a-c may be interconnected, such as by being integrally formed and/or welded together. Each ported boss 16 p may be in fluid communication with the plenum via a respective port formed in the coupling 13 and may have a nozzle (not shown) fastened therein for discharging the milling fluid onto the respective roller disk 11 a-c. Each bearing shaft 16 b may extend from the respective shirttail 16 h in a radially inclined direction toward a center of the roller-mill bit 1.

Alternatively, the roller-mill bit 1 may include a flow passage formed through the dome 15 for each roller disk 11 a-c instead of the ported bosses 16 p. Each flow passage may be in fluid communication with the plenum and may have a nozzle (not shown) fastened therein for discharging the milling fluid onto the respective roller disk 11 a-c. The flow passages may be arranged about a center of the roller-mill bit 1.

The fixed mill 12 may include a pad 12 a-c for each roller disk 11 a-c and a plurality of cutters 17 mounted to the pads. Each mill pad 12 a-c may have the shape of a conical-polyhedron including a round base 23 b and a plurality of side faces converging toward a truncated apex. The side faces of each mill pad 12 a-c may include one or more mounting faces 23 m adjacent to the apex thereof for mating with one or more complementary mounting faces of adjacent mill pads. The mill pads 12 a-c may be attached together at the mounting faces 23 m, such as by welds. Each mill pad 12 a-c may be made from a metal or alloy, such as steel.

Each bearing shaft 16 b and the respective roller disk 11 a-c may have one or more pairs of aligned grooves and each pair may form a race for receiving a set of roller bearings 18. A thrust washer 19 may be disposed between an inner face of each roller disk 11 a-c and the respective base 23 b, such as being disposed in a groove formed in the base. The roller bearings 18 and thrust washers 19 may support rotation of each disk 11 a-c relative to the respective leg 14 a-c and mill pad 12 a-c. The roller bearings 18 and thrust washers 19 may be lubricated by the milling fluid discharged from the respective ported boss 16 p.

Each mill pad 12 a-c may have a receptacle formed therein and extending from the base. An end of a respective bearing shaft 16 b may be received in the receptacle, thereby mounting the mill pads 12 a-c to the body 10 once the mill pads 12 a-c have been welded together. Each roller disk 11 a-c may be secured to the respective leg 14 a-c by entrapment between the base of the respective mill pad 12 a-c and the respective shirttail 16 h. Each roller disk may have 11 a-c may have a taper formed at a periphery of the inner face and each base 23 b may have a complementary lip formed at a periphery thereof.

Alternatively, each leg 14 a-c may have a lubricant reservoir (not shown) formed therein and a lubricant passage (not shown) extending from the reservoir to the respective roller bearing set. The lubricant may be retained within each leg 14 a-c by a pair of seals (not shown), such as o-rings, each seal positioned in a respective gland (not shown) formed in an inner surface of the respective roller disk 11 a-c. A first gland of each pair may be located adjacent to the shirttail 16 h and a second gland of each pair may be located adjacent to the respective thrust washer 19, thereby preventing leakage of lubricant from the roller bearings 18 into the wellbore 3. A pressure compensator (not shown) may be disposed in each reservoir for regulating lubricant pressure therein. An equalization passage may extend from each reservoir and through the dome 15 for operation of the respective pressure compensator to regulate the lubricant pressure to be slightly greater than bottomhole pressure. In this alternative, the thrust washers 19 may still be lubricated by the milling fluid discharged from the respective ported boss 16 p. In a variant of this alternative, the second gland may be located in the inner face of each roller disk 11 a-c such that the thrust washers 19 are also lubricated by the lubricant.

Alternatively, upper and lower edges of each shirttail 16 h may be protected from erosion and/or abrasion by respective hardfacing with a ceramic or cermet material. An outer surface of each shirttail 16 h may also be protected from erosion and/or abrasion by protective inserts secured into sockets thereof, such as by interference fit or brazing. Each protective insert may be made from a cermet. Each roller disk 11 a-c may be treated to resist erosion and/or abrasion by case hardening, such as carburization.

Each roller disk 11 a-c may have a plurality of lands formed therein, such as a heel land and a gage land. A gage row of crushers 20 may be mounted around each roller disk 11 a-c at the respective gage land. Each crusher 20 may be an insert mounted in a respective socket formed in the respective roller disk 11 a-c by an interference fit. Each crusher 20 may be made from a cermet, such as a cemented carbide, and may have a cylindrical or conical portion mounted in the respective roller disk 11 a-c and a conical or chisel portion protruding from a respective land of the respective roller disk.

A row of heel protectors 21 may be mounted around each roller disk 11 a-c at a respective heel land. Each heel protector 21 may be an insert mounted in a respective socket formed in the respective roller disk 11 a-c by an interference fit. Each protector 21 may be made from a cermet, such as a cemented carbide, and may be cylindrical.

Alternatively, the crushers 20 and/or heel protectors 21 may be capped with polycrystalline diamond (PCD). Alternatively, each crusher 20 may be a hardfaced milled tooth or each row of the crushers may include both inserts and milled teeth.

The roller-mill bit 1 may further have a junk slot 22 formed between the shirttails 16 h of each adjacent leg 14 a-c. Each junk slot 22 may be formed into the body 10, such as by milling and/or forging. Each ported boss 16 p may be located in a respective junk slot 22. Each junk slot 22 may be sized to allow passage of debris (not shown) created during milling of the frac plugs 2 into an annulus formed between the mill string and the casing or liner string 4.

FIGS. 4A-4C illustrate typical one of the mill cutters 17. To form the fixed mill 12, each mill pad 12 a-c may be dressed with the mill cutters 17. The mill cutter 17 may be a block, such as a cubic block, of cermet material. The cermet material may be a cemented carbide including a binder and carbide, such as cobalt-tungsten carbide. The cermet material may be formed into the block by sintering, such as hot pressing.

The mill cutter 17 may have a pair of opposite rectangular sides and four profiled sides connecting the rectangular sides. The profiled sides may each have rectangular end portions located adjacent to the respective rectangular sides and profiled mid portions connecting the respective end portions. Each rectangular end portion may have chamfered corners adjacent to the respective rectangular sides. Each profiled portion may have a pair of opposed trapezoidal portions converging from the respective end portions toward a center of the mill cutter 17. Each profiled portion may further have a filleted rectangular center portion connecting ends of the trapezoidal portions distal from the respective end portions. Each rectangular side may have a raised peripheral portion and a recessed interior portion. Tapered walls may connect each raised peripheral portion to the respective interior portion. Each corner of the tapered walls may be shaved.

FIGS. 4D-4F illustrate mounting of the mill cutter 17 to a typical one of the mill pads 12 a-c. The side faces of each mill pad 12 a-c may also include one or more, such as three, working faces 23 w. Each working face 23 w may have a plurality of grooves 24 formed therein for dressing the respective mill pad 12 a-c with the mill cutters 17. The grooves 24 may each be vee-shaped to facilitate a desired orientation of the mill cutters 17 therein. The desired orientation may be either the orientation illustrated in FIG. 4E or the orientation illustrated in FIG. 4F.

Each mill cutter 17 may occupy only a small fraction of a surface of the respective mill pad 12 a-c such that many cutters are necessary to dress the surface, such as greater than or equal to ten, fifteen, twenty, or thirty mill cutters. The mill cutters 17 may be mounted in the respective grooves 24, such as by brazing. To facilitate the brazing operation, several mill cutters 17 may be combined in a rod (not shown) with a tinning binder which allows a welder (person or robot) to rapidly braze the cutters on the surfaces.

Alternatively, each working face 23 w may be non-profiled and the cutters mounted thereto in a random orientation. Alternatively, each mill pad 12 a-c may be hardfaced with a ceramic or cermet material instead of having the mill cutters 17 mounted thereto.

In comparison to the prior art, when using a junk mill to drill through the frac plugs 2, excessive torque is exerted on the drilling motor causing stalling of the motor which adversely affects efficiency of the operation and shortens the service life of the motor. When using a roller cone drill bit to drill through the frac plugs 2, the bit may require replacement before all of the frac plugs have been drilled out due to bearing failure. Advantageously, the roller-mill bit 1 gains the advantages of the junk mill and the roller cone drill bit while not suffering from the drawbacks of either. The roller disks 11 a-c reduce torque while still providing stability against the casing or liner string 4. Attachment of the mill pads 11 a-c together creates additional support for the roller disks 11 a-c, thereby reducing loading on the roller bearings 18 and thrust washers 19 and extending the service lifetime thereof. Additionally, the fixed mill 12 may be replaced by grinding through the welds attaching the mill pads 11 a-c together to remove the worn fixed mill, setting new mill pads onto the bearing shafts, and welding together the new mill pads.

FIG. 5 illustrates drilling of a wellbore 25 using a hybrid roller-drag bit 26, according to another embodiment of the present disclosure. The wellbore 25 may be drilled using a drilling system 27. The drilling system 27 may include a drilling rig 27 r, a fluid handling system 27 f, a blowout preventer (BOP) 27 b, a drill string 28, and a controller, such as programmable logic controller (PLC) 27 p. The drilling rig 27 r may include a derrick 29 d, top drive 30, draw works 31, and a floor 29 f at its lower end having an opening through which the drill string 28 extends downwardly into the wellbore 25 via a wellhead 32. The BOP 27 b may be connected to the wellhead 32.

The drill string 28 may include a bottomhole assembly (BHA) 28 b and a pipe string 28 p. The pipe string 28 p may include joints of drill pipe connected together, such as by threaded couplings. The BHA 28 b may be connected to the pipe string 28 p, such as by threaded couplings, and include the roller-drag bit 26 and one or more drill collars 33. The BHA members 26, 33 may be interconnected, such as by threaded couplings. The roller-drag bit 26 may be rotated 34 r by the top drive 30 via the pipe string 28 p.

Alternatively, the BHA 28 b may include a drilling motor for rotation of the roller-drag bit 26 instead of or in addition to the top drive 30. Alternatively, the drill string may include coiled tubing instead of the pipe string 28 p. Alternatively, the BHA 28 b may further include a steering tool, such as a bent sub or rotary steering tool, and a telemetry uplink for communication with the PLC 27 p.

An upper end of the pipe string 28 p may be connected to a quill of the top drive 30. The top drive 30 may include a motor for rotating 34 r the drill string 28. The top drive motor may be electric or hydraulic. A frame of the top drive 30 may be coupled to a rail (not shown) of the derrick 29 d for preventing rotation of the top drive frame during rotation 34 r of the drill string 28 and allowing for vertical movement of the top drive with a traveling block 31 t of the draw works 31. The frame of the top drive 30 may be suspended from the derrick 29 d by the traveling block 31 t. The traveling block 31 t may be supported by wire rope 31 r woven through sheaves of the blocks 31 c,t and extending to a winch 31 w for reeling thereof, thereby raising or lowering the traveling block 31 t relative to the rig floor 29 f.

The wellhead 32 may be mounted on a casing string 35 which has been deployed into the wellbore 25 and cemented 36 therein. A lower section of the wellbore 25 may be vertical (shown) or deviated (not shown), such as slanted or horizontal.

The fluid handling system 27 f may include a mud pump 37, a drilling fluid reservoir, such as a pit 38 or tank, a solids separator, such as a shale shaker 39, a pressure sensor 40, one or more flow lines, such as a return line 41 r, a supply line 41 s, and a feed line 41 f, and a stroke counter 42. A first end of the return line 41 r may be connected to a flow cross 43 mounted on the wellhead 32 and a second end of the return line may be connected to an inlet of the shaker 39. A lower end of the supply line 41 s may be connected to an outlet of the mud pump 37 and an upper end of the supply line may be connected to an inlet of the top drive 30. The pressure sensor 40 may be assembled as part of the supply line 41 s. A first end of the feed line 41 f may be connected to an outlet of the pit 38 and a second end of the feed line may be connected to an inlet of the mud pump 37.

The pressure sensor 40 may be in data communication with the PLC 27 p and may be operable to monitor standpipe pressure. The stroke counter 42 may also be in data communication with the PLC 27 p and may be operable to monitor a flow rate of the mud pump 12. The PLC 27 p may also be in communication with a hook load cell clamped to the wire rope 31 r, and a position sensor of the winch 31 w for monitoring depth of the BHA 28 b. The PLC 27 p may further be in communication with a torque sensor and tachometer of the top drive 30.

The mud pump 37 may pump drilling fluid 44 from the pit 38, through the supply line 41 s, and to the top drive 30. The drilling fluid 44 may include a base liquid. The base liquid may be refined or synthetic oil, water, brine, or a water/oil emulsion. The drilling fluid 44 may further include solids dissolved or suspended in the base liquid, such as organophilic clay, lignite, and/or asphalt, thereby forming a mud.

The drilling fluid 44 may flow from the supply line 41 s and into a bore of the pipe string 28 p via the top drive 30. The drilling fluid 44 may flow down the pipe string 28 p, through a bore of the BHA 28 b, and exit the roller-drag bit 26, where the fluid may circulate cuttings away from the bit and return the cuttings up an annulus 45 formed between an inner surface of the casing 35 or the wellbore 25 and an outer surface of the drill string 28. The returns 46 (drilling fluid 44 plus cuttings) may flow up the annulus 45, to the wellhead 32, and exit the wellhead through the flow cross 43. The returns 46 may continue through the return line 41 r and into the shale shaker 39 and be processed thereby to remove the cuttings, thereby completing a cycle. As the drilling fluid 44 and returns 46 circulate, the drill string 28 may be rotated 34 r by the top drive 30 and lowered 34 a by the traveling block 31 t, thereby extending the wellbore 25 to a hydrocarbon-bearing formation or to a depth sufficient for geothermal power generation.

FIGS. 6 and 7A illustrate the roller-drag bit 26. FIG. 7B illustrates mounting of a fixed cutting structure 47 to the roller-drag bit 26. The roller-drag bit 26 may include a body 48, a plurality of roller disks 49 a-c, a plurality of crushers 55 g,n, and the fixed cutting structure 47. The roller disks 49 a-c, crushers 55 g,n, and fixed cutting structure 47 may form a lower cutting face of the roller-drag bit 26. The roller disks 49 a-c and crushers 55 g,n may be located at an outer portion of the cutting face and the fixed cutting structure 47 may be located at an inner portion of the cutting face.

The body 48 may have an upper coupling 50, a lower leg 51 for each roller disk 49 a-c, and a dome 52 formed between the legs. The body 48 and the roller disks 49 a-c may each be made from a metal or alloy, such as steel. The body 48 may be made by attaching three forgings together, such as by welding. The legs 51 may be equally spaced around the body, such as three at one hundred twenty degrees. The upper coupling 50 may be a threaded pin for connection to the drilling motor or drill pipe. A bore may be formed through the coupling 50 and extend to a plenum (not shown) formed adjacent to the dome 52.

Each leg 51 may have an upper shoulder 51 s, a mid shirttail 51 h, a lower bearing shaft 51 b, and a ported boss 51 p. The shoulder 51 s, shirttail 51 h, ported boss 51 p, and bearing shaft 51 b of each leg 51 may be interconnected, such as by being integrally formed and/or welded together. Each ported boss 51 p may be in fluid communication with the plenum via a respective port formed in the coupling 50 and may have a nozzle (not shown) fastened therein for discharging the drilling fluid 44 onto the respective roller disk 49 a-c. Each bearing shaft 51 b may extend from the respective shirttail 51 h in a radially inclined direction toward a center of the roller-drag bit 26.

Alternatively, the roller-drag bit 26 may include a flow passage formed through the dome 52 for each roller disk 49 a-c instead of the ported bosses 51 p. Each flow passage may be in fluid communication with the plenum and may have a nozzle (not shown) fastened therein for discharging the milling fluid onto the respective roller disk 49 a-c. The flow passages may be arranged about a center of the roller-drag bit 26.

The fixed cutting structure 47 may include a center hub 47 h, a blade 47 a-c for each roller disk 49 a-c, and a plurality of shear cutters 53 mounted to each blade and embedded in the hub. The hub 47 h and the blades 47 a-c may be monolithic and each blade may extend radially outward from the hub. Each blade 47 a-c may have an outer face 47 f formed for mating with a complementary inner face 49 f of an adjacent roller disk 49 a-c. The shear cutters 53 may be mounted, such as by brazing, in respective pockets formed along leading edges of the blades 47 a-c. The shear cutters 53 may be embedded, such as by brazing, in respective receptacles formed in a bottom of the center hub 47 h.

Additionally, the fixed cutting structure 47 may further include a plurality of backup shear cutters (not shown). The backup shear cutters may be mounted in pockets formed along bottoms of the blades 47 a-c, such as by brazing. Each backup shear cutter may be aligned or slightly offset from a respective (leading) shear cutter 53. Additionally, the fixed cutting structure 47 may further include a plurality of depth-of-cut (DOC) limiters (not shown). The DOC limiters may be mounted in sockets formed in bottoms of the blades 47 a-c, such as by brazing or interference fit. The DOC limiters may be bullet shaped or ovoid and made from any of the materials discussed above for the crushers 20.

The center hub 47 h and blades 47 a-c may be made from a composite material, such as a ceramic and/or cermet powder infiltrated by a metallic binder, or may be metallic, such as being made from steel, and may be hardfaced. Each shear cutter 53 may include a superhard cutting table, such as polycrystalline diamond, attached to a hard substrate, such as a cermet, thereby forming a compact, such as a polycrystalline diamond compact (PDC). The cermet may be a carbide cemented by a Group VIIIB metal, such as cobalt-tungsten carbide. The substrate and the cutting table may each be solid and cylindrical and a diameter of the substrate may be equal to a diameter of the cutting table.

The blades 47 a-c may be equally spaced around the hub 47 h, such as three at one hundred twenty degrees. A void may be formed between each adjacent blade 47 a-c to accommodate the drilling fluid discharged from the ported bosses 51 p. A bottom of each blade 47 a-c may be contoured. The contoured bottoms may be flush or almost flush with lands of the roller disks 49 a-c at the outer faces 47 f. The contoured bottoms may incline toward the dome 52 along the blades 47 a-c approaching the hub 47 h and may flatten as the bottoms reach the hub. A thickness of the blades 47 a-c and the hub 47 h may correspond to a diameter of the roller disks 49 a-c, such as being equal to or almost equal to, thereby forming a gap between a top of the fixed cutting structure 47 and the dome 52. The diameter of each roller disk 49 a-c may be equal. A width of each blade 47 a-c may range between one-half the diameter of the roller disks 49 a-c and the diameter of the roller disks.

Each bearing shaft 51 b and the respective roller disk 49 a-c may have one or more pairs of aligned grooves and each pair may form a race for receiving a set of roller bearings (not shown). A thrust washer (not shown) may be disposed between each inner face 49 f and the respective outer face 47 f, such as being disposed in a groove (not shown) formed in the outer face. The roller bearings and thrust washers may support rotation of each roller disk 49 a-c relative to the respective leg 51 and the fixed cutting structure 47. The thrust washers may be lubricated by the drilling fluid 44 discharged from the ported bosses 51 p.

Each leg 51 may have a lubricant reservoir (not shown) formed therein and a lubricant passage (not shown) extending from the reservoir to the respective roller bearing set. The lubricant may be retained within each leg 51 by a pair of seals (not shown), such as o-rings, each seal positioned in a respective gland (not shown) formed in an inner surface of the respective roller disk 49 a-c. A first gland of each pair may be located adjacent to the shirttail 51 h and a second gland of each pair may be located adjacent to the respective thrust washer, thereby preventing leakage of lubricant from the roller bearings into the wellbore 25. A pressure compensator (not shown) may be disposed in each reservoir for regulating lubricant pressure therein. An equalization passage may extend from each reservoir and through the dome 52 for operation of the respective pressure compensator to regulate the lubricant pressure to be slightly greater than bottomhole pressure.

Alternatively, the second gland may be located in the inner face of each roller disk 49 a-c such that the thrust washers are also lubricated by the lubricant.

The roller-drag bit 26 may further include a threaded fastener 54 for each roller disk 49 a-c and blade 47 a-c for mounting the fixed cutting structure 47 to the shirttails 51 h. Each blade 47 a-c may have a receptacle formed therein and extending from the respective outer face 47 f. Each blade 47 may also have a socket (not shown) formed therein and extending from an end of the respective receptacle. Each receptacle may be oversized relative to the respective bearing shaft 51 b facilitate positioning of the fixed cutting 47 structure onto. Each shirttail 51 h and bearing shaft 51 b may have a bore formed therein for receiving a shaft of the respective threaded fastener therethrough. Each shirttail 51 h may have a countersunk or counterbored opening adjacent to the respective bore for receiving a head of the respective threaded fastener 54. Each blade socket may be threaded for receiving an end of the shaft of the respective threaded fastener 54, thereby mounting the respective blade 47 a-c to the respective bearing shaft 51 b and securing each roller disk 49 a-c to the respective leg 51 by entrapment between the outer face 47 f of the respective blade and the respective shirttail 51 h.

Alternatively, upper and lower edges of each shirttail 51 h may be protected from erosion and/or abrasion as discussed above for the shirttails 16 h.

Each roller disk 49 a-c may have a plurality of lands formed therein, such as a heel land, a gage land, and an inner land for the roller disks 49 a,b and a heel land and a gage land for the roller disk 49 c. A gage row of crushers 55 g may be mounted around each roller disk 49 a-c at the respective gage land. An inner row of crushers 55 n may be mounted around each roller disk 49 a,b at the respective inner land. Each crusher 55 g,n may be an insert mounted in a respective socket formed in the respective roller disk 49 a-c by an interference fit. Each crusher 55 g,n may be made from a cermet, such as a cemented carbide, and may have a cylindrical or conical portion mounted in the respective roller disk 49 a-c and a conical or chisel portion protruding from a respective land of the respective roller disk.

A row of heel protectors 56 may be mounted around each roller disk 49 a-c at a respective heel land. Each heel protector 56 may be an insert mounted in a respective socket formed in the respective roller disk 49 a-c by an interference fit. Each protector 56 may be made from a cermet, such as a cemented carbide, and may be cylindrical.

Alternatively, the crushers 55 g,n and/or heel protectors 56 may be capped with polycrystalline diamond (PCD). Alternatively, each crusher 55 g,n may be a hardfaced milled tooth or each row of the crushers may include both inserts and milled teeth.

FIGS. 8 and 9A illustrate a second hybrid roller-drag bit 57, according to another embodiment of the present disclosure. The second hybrid roller-drag bit 57 may be assembled as part of the BHA 28 b instead of the hybrid roller-drag bit 26. The second hybrid roller-drag bit 57 may include a body 58, a plurality of roller disks 59 a,b, the crushers 55 g,n, a fixed cap 60, a stem 63, and a plurality of the shear cutters 53. The roller disks 59 a,b, crushers 55 g,n, fixed cap 60, and shear cutters 53 may form a lower cutting face of the second roller-drag bit 57. The roller disks 59 a,b and crushers 55 g,n may be located at an outer portion of the cutting face and the fixed cap 60 and shear cutters 53 may be located at an inner portion of the cutting face.

The body 58 may have the upper coupling 50, a lower leg 61 for each roller disk 59 a,b, and a dome 62 formed between the legs. The body 58 and the roller disks 59 a,b may each be made from a metal or alloy, such as steel. The body 58 may be made by attaching three forgings together, such as by welding. The legs 61 may be equally spaced around the body, such as three at one hundred twenty degrees. A bore may be formed through the coupling 50 and extend through the dome 62. The bore may be centrally positioned about the second hybrid roller-drag bit 57.

Each leg 61 may have an upper shoulder 61 s, a mid shirttail 61 h, and a lower bearing shaft 61 b. The shoulder 61 s, shirttail 61 h, and bearing shaft 61 b of each leg 61 may be interconnected, such as by being integrally formed and/or welded together. Each bearing shaft 61 b may extend from the respective shirttail 61 h in a radially inclined direction toward a center of the second hybrid roller-drag bit 57. Each bearing shaft 61 b and the respective roller disk 59 a,b may have one or more pairs of aligned grooves and each pair may form a race for receiving a set of roller bearings 64. The roller bearings 64 may support rotation of each roller disk 59 a,b relative to the respective leg 61 and the fixed cap 60.

Each leg 61 may have a lubricant reservoir (not shown) formed therein and a lubricant passage (not shown) extending from the reservoir to the respective roller bearing set. The lubricant may be retained within each leg 61 by a seal (not shown), such as an o-ring, positioned in a gland (not shown) formed in an inner surface of the respective roller disk 59 a,b. A pressure compensator (not shown) may be disposed in each reservoir for regulating lubricant pressure therein. An equalization passage may extend from each reservoir and through the dome 62 for operation of the respective pressure compensator to regulate the lubricant pressure to be slightly greater than bottomhole pressure.

Each roller disk 59 a,b may be secured to the respective leg 61 by a plurality of balls (not shown) received in a race formed by aligned grooves (not shown) in each roller disk and the respective bearing shaft 61 b. The balls may be fed to each race by a ball passage (not shown) formed in each leg 61 and retained therein by a respective ball plug (not shown). Each ball plug may be attached to the respective leg 61, such as by welding.

The stem 63 may be made from thick-walled pipe and a metal or alloy, such as steel. An upper end of the stem 63 may be threaded and may be received in a threaded socket formed in the dome 62 adjacent to the bore, thereby mounting the stem to the body 58. The stem 63 may extend downwardly from the dome 62 and have a length corresponding to a diameter of the roller disks 59 a,b. The stem 63 may have a port formed through a wall of thereof for each roller disk 59 a,b. Each port may discharge drilling fluid 44 onto the respective roller disk 59 a,b. The second hybrid roller-drag bit 57 may further include a nozzle 65 for each port. Each nozzle 65 may be mounted to the stem, such as by having a thread formed in an outer surface thereof engaged with a respective thread formed in a wall of the stem adjacent to the respective port. Each nozzle 65 may be made from an erosion resistant material, such as a cermet.

The fixed cap 60 may be made from any of the materials discussed above for the blades 47 a-c and hub 47 h. The fixed cap 60 may have a threaded socket formed in an upper portion thereof for engagement with a threaded lower end of the stem 63, thereby mounting the fixed cap onto the stem. The fixed cap 60 may have a concave bottom and the shear cutters 53 embedded, such as by brazing, in respective receptacles formed therein.

Each roller disk 59 a,b may have a plurality of lands formed therein, such as a heel land, a gage land, and an inner land. A gage row of the crushers 55 g may be mounted around each roller disk 49 a-c at the respective gage land. An inner row of the crushers 55 n may be mounted around each roller disk 59 a,b at the respective inner land. A row of the heel protectors 56 may be mounted around each roller disk 59 a,b at a respective heel land.

FIG. 9B illustrates a third hybrid roller-drag bit 66, according to another embodiment of the present disclosure. The third roller-drag bit 66 may be similar to the second roller-drag bit 66 except for having a bladed cap 67 instead of the fixed cap 60 and accommodating modifications therefor to the stem 63 and the dome 62. The bladed cap 67 may be made from any of the materials discussed above for the blades 47 a-c and hub 47 h.

The modified dome may have a socket formed therein adjacent to the bore for receiving an upper end of the modified stem. Once the dome and stem sockets have been engaged, the modified stem may be mounted thereto by a weld (not shown). The bladed cap 67 may have a socket formed in an upper portion thereof for engagement with a lower end of the modified stem. Once the cap and stem sockets have been engaged, the bladed cap 67 may be oriented and then mounted thereto by a weld (not shown).

The bladed cap 67 may be made from any of the materials discussed above for the blades 47 a-c and hub 47 h. The bladed cap 67 may include a center hub 67 h, a blade 67 a-c for each roller disk 59 a,b, and a plurality of shear cutters 53 mounted to each blade and embedded in the hub. The hub 67 h and the blades 67 a-c may be monolithic and each blade may extend radially outward from the hub. The shear cutters 53 may be mounted, such as by brazing, in respective pockets formed along leading edges of the blades 67 a-c. The shear cutters 53 may be embedded, such as by brazing, in respective receptacles formed in a bottom of the center hub 67 h. A bottom of each blade 67 a-c may be contoured. The contoured bottoms may incline toward the dome 62 along the blades 67 a-c approaching the hub 67 h and may flatten as the bottoms reach the hub.

Additionally, the bladed cap 67 may further include backup shear cutters and/or DOC limiters (not shown) as discussed above for the fixed cutting structure 47.

The blades 67 a-c may be equally spaced around the hub 67 h, such as three at one hundred twenty degrees. The roller disks 59 a,b may be sized such that a cavity is formed between each adjacent roller disk. The bladed cap 67 may be oriented such that each blade 67 a-c extends into the respective cavity. The blades 67 a-c may extend into the cavities to create an overlap in a cutting profile of the third roller-drag bit 66. The blades may extend to an effective diameter greater than or equal to one-third, one-half, or two-thirds a gage diameter of the third roller-drag bit 66. A void may be formed between each adjacent blade 67 a-c to accommodate roller disks 59 a,b.

Alternatively, the dome and the upper portion of the stem may not be modified and connected together by the threaded connection.

As used herein, reference to a wellbore may be either to a cased or lined section of the wellbore or to an open-hole section of the wellbore.

While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope of the invention is determined by the claims that follow. 

1. A roller-mill bit for use in a wellbore, comprising: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller disks, each roller disk secured to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller disk; and a fixed mill mounted to the bearing shafts and comprising a pad for each roller disk, wherein each pad is dressed with a cermet material.
 2. The roller-mill bit of claim 1, wherein: the roller disks, crushers, and fixed mill form a lower cutting face of the roller-mill bit, and the roller disks and crushers are located at an outer portion of the cutting face and the fixed mill is located at an inner portion of the cutting face.
 3. The roller-mill bit of claim 2, wherein: each leg further has a shirttail, and each bearing shaft extends from a respective shirttail in a radially inclined direction toward a center of the roller-mill bit.
 4. The roller-mill bit of claim 1, wherein: each mill pad has a round base and a plurality of side faces converging toward a truncated apex, at least one of the side faces of each mill pad is a mounting face, and the mill pads are attached together at the mounting faces.
 5. The roller-mill bit of claim 4, further comprising: a set of roller bearings disposed between each bearing shaft and the respective roller disk; and a thrust washer disposed between each roller disk and the respective base.
 6. The roller-mill bit of claim 5, wherein: each leg further has a shirttail and a ported boss, a junk slot is formed between the shirttails of each adjacent leg, and each ported boss is located in a respective junk slot.
 7. The roller-mill bit of claim 6, wherein the roller bearings and thrust washers are configured to be lubricated by fluid discharged from the ported bosses.
 8. The roller-mill bit of claim 5, wherein each leg has a lubricant reservoir formed therein and a lubricant passage extending from the reservoir to the respective roller bearing set.
 9. The roller-mill bit of claim 4, wherein: each mill pad has a receptacle formed therein and extending from the base, and an end of a respective bearing shaft is received in the receptacle, thereby mounting the mill pads to the body.
 10. The roller-mill bit of claim 9, wherein: each leg further has a shirttail, and each roller disk is secured to the respective leg by entrapment between the base of the respective mill pad and the respective shirttail.
 11. The roller-mill bit of claim 9, wherein each roller disk has a taper formed at a periphery of an inner face thereof and each base has a complementary lip formed at a periphery thereof.
 12. The roller-mill bit of claim 1, wherein the roller-mill bit comprises three legs and three roller disks.
 13. The roller-mill bit of claim 1, wherein the body further has a dome formed between the legs, a bore formed through the coupling and extending to a plenum formed adjacent to the dome, and a passage formed through the dome for each roller disk.
 14. The roller-mill bit of claim 1, wherein the cermet material is a plurality of cutter blocks brazed to the pad.
 15. The roller-mill bit of claim 14, wherein the cutter blocks are brazed into grooves formed in the pad at a desired orientation.
 16. The roller-mill bit of claim 14, wherein: each cutter block has a pair of opposite rectangular sides and four profiled sides connecting the rectangular sides, the profiled sides each have rectangular end portions located adjacent to the respective rectangular sides and profiled mid portions connecting the respective end portions, and each rectangular side has a raised peripheral portion and a recessed interior portion.
 17. A method of drilling out a plug using the roller-mill bit of claim 1, assembling the roller-mill bit as part of a mill string; deploying the mill string into a casing or liner string set in the wellbore to the plug set in the casing or liner string; and injecting milling fluid through the mill string, rotating the roller-mill bit, and engaging the roller-mill bit with the plug, thereby drilling out the plug.
 18. A roller-drill bit for drilling a wellbore, comprising: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller disks, each disk secured to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller disk; and a fixed cutting structure mounted to the bearing shafts and including: a center hub; a blade for each roller disk, and plurality of shear cutters mounted to each blade.
 19. The roller-drill bit of claim 18, wherein the fixed cutting structure further has a plurality of shear cutters embedded in the hub.
 20. The roller-drill bit of claim 18, wherein: each leg further has a shirttail, the roller-drill bit further comprises a threaded fastener for each roller disk and blade, each shirttail and bearing shaft has a bore formed therein receiving a shaft of the respective threaded fastener therethrough, each shirttail has an opening adjacent to the respective bore receiving a head of the respective threaded fastener, and each blade has a threaded socket formed therein and receiving an end of the shaft of the respective threaded fastener, thereby mounting the respective blade to the respective bearing shaft and securing each roller disk to the respective leg.
 21. A roller-drill bit for drilling a wellbore, comprising: a body having a coupling formed at an upper end thereof, a plurality of lower legs, a dome formed between the legs, and a bore formed through the coupling and the dome; a plurality of roller disks, each disk secured to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller disk; and a stem mounted to the dome adjacent to the bore and having a port formed through a wall thereof for each roller disk; a fixed cap mounted to a lower end of the stem; and one or more shear cutters embedded in or mounted to the fixed cap.
 22. The roller-drill bit of claim 21, wherein: the fixed cap has a center hub and a blade for each roller disk, a cavity is formed between each adjacent roller disk, and the fixed cap is oriented such that each blade extends into the respective cavity. 